1. Field of the Invention
The present invention relates to an image forming apparatus and a method of controlling transfer current.
2. Description of Related Art
In a conventional electrophotographic image forming apparatus, a transfer material or an intermediate transfer body passes between a photoreceptor and a transfer member. During the passage, a toner image on the photoreceptor is transferred to the transfer material or intermediate transfer body by a transfer bias applied to the transfer member. The transfer bias is generally controlled under constant voltage control or constant current control.
The transfer member is generally composed of a transfer roller. The transfer roller is made of a material having an electric resistance (hereinafter, referred to as a resistance) highly dependent on the environment. Various techniques to address environmental change have been proposed.
For example, Patent Literature 1 (Japanese Patent Application Laid-open Publication No. 08-114989) describes a technique to perform constant current control and constant voltage control according to whether the resistance value of the contact transfer member measured by a resistance measuring means is higher than a predetermined reference value. Moreover, Patent Literature 1 describes a static charge eliminator which is configured to be switched to a DC bias applied mode at low temperature and low humidity, a grounding mode at room temperature and room humidity; and a floating mode at high temperature and high humidity.
Patent Literature 2 (Japanese Patent application Laid-open Publication No. 2000-66536) describes a technique of holding plural levels of potential applied to a static charge eliminating member and switching the levels of potential applied to the static charge eliminating member according to voltage detected with the constant current bias applied to the transfer roller.
By the way, inflowing current varies on the resistance of the transfer member. In the case where the transfer member is under constant voltage control, therefore, if the transfer member continues to be subjected to constant voltage, changing resistance of the transfer member changes transfer properties, thus changing the image density. Accordingly, it is necessary to reset optimal applied voltage at proper intervals. As the method of controlling the same, ATVC (active transfer voltage control) is well known. The ATVC determines voltage to be applied to the transfer member so that the transfer current flowing through the transfer member be a previously determined optimal transfer current.
However, if continuous printing is performed by a print job for a lot of sheets, the resistance of the transfer member decreases to cause excess current, thus degrading the transfer properties (see FIGS. 13A and 13B). To solve this problem, the ATVC may be performed for each certain number of prints or each certain change in temperature. However, the ATVC takes long time, thus causing reduction in productivity.
The techniques of the aforementioned Patent Literatures 1 and 2 cannot cope with the change in density caused by excess current due to changing resistance of the transfer member under constant voltage control.